Keywords:ab initio calculation, MgO-SiO2 system, incongruent melting, multi-megabar
Magnesium silicates are thought to be the major components of the mantle of terrestrial planets and the core of giant planets (Guillot, 1999; Seager et al., 2007). However, the thermodynamic phase equilibrium in the MgO-SiO2 system is still not well studied at multi-megabar, including melting relations. A recent laser shock experience reported two discontinuous phase changes in MgSiO3 at 300-400 GPa (Spaulding et al., 2012), but an ab initio molecular dynamics study identified no clear transition in MgSiO3 liquid (Militzer, 2013). Boates and Bonev (2013), on the other hand, examined a decomposition reaction of liquid MgSiO3 into solid MgO and liquid SiO2 and reported that liquid MgSiO3 is dissociated at ~300 GPa. This result implies a possible incongruent melting. However, the reaction they considered is too simple and unrealistic. The detailed phase diagram in the MgO-SiO2 system is therefore required to be clarified at multi-megabar. In this study, we perform ab initio free energy calculations based on the thermodynamic integration method (Kirkwood, 1935) and determine the melting phase relation in this binary system.